Aerated Concrete With High Volume of Slag (GGBFS)

It has been traditionally practiced to evaluate the Concrete Contractor Resida CA through its mechanical, physical, and functional properties. However, often we disregard, that these properties are the result of the ‘internal architecture’ that makes up the concrete. Microstructure-property relationship is at the heart of modern material science. Concrete is highly heterogeneous and has very complex microstructure. Therefore, it is very difficult to constitute a realistic model of its microstructure in order to understand the behavior of the material. The microstructure of concrete also changes with age, cement content, the water: cement ratio, curing, chemical admixtures, and incorporation of pozzolan material (slag, fly ash, etc.).

Furthermore many concrete in service are subject to deterioration by various chemical and physical processes, all of which modify their internal structures as well as their end-use properties. Scanning electron microscopy (SEM) has been a primary tool in the investigation of the complex internal structure of concretes and hydrated cement pastes for many years. While the ‘internal architecture’ of concrete can be studied by various techniques, no other technique can provide the depth and breadth of information available with SEM.

In aerated concrete, pore system can be divided into three regions. One of them consists of air pores with a radius of 50 to 500?m introduced by hydrogen gas during the manufacturing process. Another region is featured by micro-capillaries of 50 nm or less, which is the gap of the hydration products developed in the wall between the air pores. Besides these two regions, there are very few pores with size of 50 nm to 50?m, which is referred as macro-capillaries (Alexanderson 1979; Prim et al. 1983; Tada et al. 1983).

Even though the air void system remains largely identical, there still exists some difference in the structure of autoclaved aerated concrete (AAC) and non-autoclaved aerated concrete (NAAC). This is caused by mainly due to the variation in the hydration products. On autoclaving, a part of fine siliceous material reacts chemically with calcareous material like lime and lime liberated by cement hydration, forming a microcrystalline structure with much lower specific surface, which would result in higher strength. On the other hand, non-autoclaved aerated concrete (NAAC) has a larger volume of fine pores due the presence of excessive pore water.

In general microstructural changes occurs due to the variance in exposure conditions, composition variations, and age. These changes will significantly affect the properties of aerated concrete. Non-autoclaved aerated concrete (NAAC) undergoes changes in structure with time whereas autoclaved aerated concrete are practically stable. There are also clear indications of the existence of a transition zone at the void-paste interface. However, the transition zone in aerated concrete is less porous compared to normal concrete. This is due the constriction of the matrix by the voids and the unlimited space available for hydration as well as for bleed water to move about.

Therefore, at least a nodding acquaintance with the internal architecture of aerated concrete in relation with the compressive strength would be beneficial to all who deal with concrete properties and with concrete behavior in service. This is particularly true for the expanding community of those engaged in developing mathematical models of concrete and of concrete durability.

Experimental Program

The experimental work comprises of compressive strength test and SEM studies on two types aerated concrete mixes, which was air cured and also exposed to natural weather and seawater. Cement base and slag replaced matrix were prepared to assist the comparative studies on the effect of slag on aerated concrete. Ordinary Portland cement of was used throughout the experimental investigation. The OPC used complies with the requirements in ASTM C150 (1992). Ground granulated blast furnace slag used was in accordance with ASTM C989 (1989). The slag activity index was 100. The sand used was sieved to the fineness of passing 600?m. Aluminum powder was used as expanding agent to produce air bubbles in the mix, and superplasticizer was used to enhance the early strength of the material. Cubes with the size of 70.6 x 70.6 x 70.6 mm was prepared to study the compressive strength. Compressive strength was tested at the age of 14 days, 28 days, 90 days, and 180 days. For the microstructural investigation, broken specimens with the size of about 10 mm were used. The specimens were mounted on a metal stubs, sputter-coated with gold before subjecting to the scanning electron microscopic. The specimen was coated, in order to transform it from non-conducive material into conducive material. The images of microstructure were taken at the age of 14 days, and 180 days for all three types of curing conditions.

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